The present invention relates to a semiconductor laser device including two semiconductor laser elements and a wire bonding method for the device.
Conventionally, there has been a semiconductor laser device in which one semiconductor laser element and a monitoring use photodiode (hereinafter referred to as a monitoring PD) for monitoring an output of the semiconductor laser element are arranged on a metallic stem. However, in order to read information from a recorded medium such as a CD (compact disc) and a DVD (digital versatile disk), there is needed a semiconductor laser device that emits two kinds of laser light of different-wave lengths by means of two semiconductor laser elements.
Accordingly, there can be considered a semiconductor laser device as shown in FIG. 12 where two semiconductor laser elements and a monitoring PD for monitoring the output of the semiconductor laser element are arranged. FIG. 12 shows a perspective view of the inside of this semiconductor laser device with its cap removed. It is to be noted that this semiconductor laser device is shown for facilitating the explanation of this invention and is not the prior art.
As shown in FIG. 12, this semiconductor laser device includes a metallic stem 200 having an eyelet 201 and a heat radiation base 202 which are integrally formed. Lead pins 221 through 223 are mounted on the eyelet 201 of the stem 200 so that one end penetrates the eyelet 201 of the stem 200, and one end of a lead pin 224 is electrically connected as a common electrode to the eyelet 201. The lead pins 221 through 223 are fixed to the eyelet 201 with a low melting point glass and electrically insulated with respect to the stem 200. The eyelet 201 has an outer diameter of 5.6 mm, and the lead pins 221 through 224 constructed of a columnar metal having a diameter of 0.4 mm are arranged at regular intervals of 90 degrees in a circle of a diameter of 2 mm.
A silicon sub-mount (hereinafter referred to as an Si sub-mount) 260 is die-bonded to the heat radiation base 202 formed integrally with the eyelet 201 with a conductive paste (not shown). Two semiconductor laser elements 231 and 232 are die-bonded onto the silicon sub-mount 260 with a brazing material (not shown) made of an Auxe2x80x94Sn alloy. The die bonding surface of the Si sub-mount 260 is covered with a metal, providing a common electrode of the semiconductor laser elements 231 and 232. The common electrode on the surface of the Si sub-mount 260 is connected to the heat radiation base 202 via metal wires 252 and 254, respectively. On the other hand, upper electrodes of the semiconductor laser elements 231 and 232 are connected to the lead pins 221 and 222 via metal wires 251 and 253, respectively. A monitoring PD 240 is die-bonded to a recess 201b formed on the eyelet 201 of the stem 200 with a conductive paste (not shown), and an upper electrode of the monitoring PD 240 is connected to an end surface 223a of the lead pin 223 via a metal wire 255.
The two semiconductor laser elements 231 and 232 are provided particularly by a combination of an InGaAlP based semiconductor laser element 231 that emits red laser light (having a wavelength of 630 nm to 680 nm) and an AlGaAs based semiconductor laser element 232 that emits infrared laser light (having a wavelength of 760 nm to 850 nm).
It is required to die-bond the semiconductor laser elements 231 and 232 onto the Si sub-mount 260 by using a brazing material (Auxe2x80x94Sn alloy, for example) whose melting point is sufficiently higher than a temperature of 80xc2x0 C., which is the upper limit of the normal use temperature range so as not to move the relative positions of the light emitting points of the two semiconductor laser elements 231 and 232 in operation. If the semiconductor laser elements 231 and 232 are die-bonded directly to the metallic heat radiation base 202, then there is the problem that an intense stress is applied to the semiconductor laser elements 231 and 232 due to a difference in the linear expansion coefficient of the metal and the semiconductor, consequently destroying and deteriorating the crystal. Therefore, it is indispensable to perform the die bonding to the Si sub-mount 260.
The semiconductor laser device having two semiconductor laser elements shown in FIG. 12 has the problem of complicated structure, and the processes of die-bonding the monitoring PD 240 and the Si sub-mount 260 increase cost.
Accordingly, it can be considered to simplify the fabricating processes by forming a monitoring PD on the surface of the Si sub-mount and eliminating the die bonding process of the monitoring PD. If the above-mentioned structure is adopted, then the electrode surface of the monitoring PD becomes parallel to the electrode surfaces of the two semiconductor laser elements and the electrode surface formed on the surface of the Si sub-mount. The wire bonding cannot easily be performed unless the surfaces of the electrodes of the semiconductor laser elements and the monitoring PD and the surfaces of the lead pins to which metal wires are to be bonded are parallel to one another when connecting the electrodes of these semiconductor laser elements and the monitoring PD with the lead pins by way of metal wires. This will be described below on the basis of the semiconductor laser device of the construction shown in FIG. 12 (monitoring PD is assumed to be formed on the surface of the Si sub-mount).
In this semiconductor laser device, the two semiconductor laser elements 231 and 232 are connected to the lead pins 221 and 222, respectively, located on both sides. Accordingly, there is only the lead pin 223 that is located on the upper side in FIG. 12 and is able to be connected to the electrode of the monitoring PD formed on the surface of the Si sub-mount. In this case, there is the problem that almost no surface parallel to the electrode of the monitoring PD to be formed on the Si sub-mount 260 exists since the tip of the lead pin 223 is not protruding from a surface 201a of the eyelet 201. As a method for solving this problem, it can also be considered to provide a recess around the lead pin 223 on the eyelet 201 to expose the lead pin 223 and perform die-bonding to the outer peripheral surface of the cylindrical lead pin 223. However, such a recess may penetrate the eyelet 201 to disable the sealing of the inside with a cap (not shown), which would cause a problem that the semiconductor laser elements easily deteriorate.
When wire-bonding the end surface 223a of the lead pin 223 to the electrode of the monitoring PD formed on the Si sub-mount 260, the end surface 223a of the lead pin 223 and the electrode surface of the monitoring PD are perpendicular to each other, and therefore, it has been difficult to connect the surfaces together by the conventional wire bonding method. The reason for the above will be described below with reference to FIG. 13 through FIG. 19, which show the wire bonding processes of the semiconductor laser device of FIG. 12.
First of all, the wire bonding method for connecting the electrode surface of the monitoring PD 240 of the semiconductor laser device 200 shown in FIG. 12 with the end surface 223a of the lead pin 223 by way of a metal wire will be described with reference to FIG. 13 through FIG. 18.
As shown in FIG. 13, a bonding head 70 has a capillary 71 attached to the tip of a capillary holder 72 and a wire clamp 73, and the capillary 71 and the wire clamp 73 move in such a manner as an integrated body. The capillary 71 has a tip diameter of about 200 xcexcm and operates to guide a metal wire 50 kept linear. A gold wire having a diameter of 25 xcexcm is used as this metal wire 50, and a ball 50a is formed by arc discharge or the like at the tip of the metal wire 50 that protrudes from the tip of the capillary 71.
Next, the bonding head 70 is moved down as shown in FIG. 14 to bring the ball 50a (shown in FIG. 13) in contact with the electrode surface of the monitoring PD 240, and supersonic vibrations are applied to the ball 50a to connect the ball 50a to the electrode of the monitoring PD 240 (the point to which this ball 50a is connected is referred to as a xe2x80x9cfirst bondxe2x80x9d).
Next, the bonding head 70 is moved up with the wire clamp 73 opened as shown in FIG. 15 to draw the metal wire 50, while the stem 200 is properly turned around an axis perpendicular to the axial direction of the capillary 71 to set the bonding surface 223a of the lead pin 223 perpendicular to the axial direction of the capillary 71.
Next, as shown in FIG. 16, the bonding head 70 is moved along a plane parallel to the bonding surface 223a of the lead pin 223 so as to locate the bonding surface 223a of the lead pin 223 perpendicularly below the capillary 71. If the electrode surface of the monitoring PD 240 and the bonding surface 223a of the lead pin 223 are not located in an identical plane with respect to the metal wire 50 guided by the capillary 71 in this stage, then it is proper to move the stem 200 so that the bonding surface 223a of the lead pin 223 is located on the axis of the capillary 71.
As shown in FIG. 17, the bonding head 70 is moved down again to bring the metal wire 50 in contact with the bonding surface 223a of the lead pin 223, and supersonic vibrations are applied to the metal wire 50 to connect the metal wire 50 to the bonding surface 223a of the lead pin 223 (the point to which this metal wire is connected is referred to as a xe2x80x9csecond bondxe2x80x9d).
Finally, as shown in FIG. 18, the metal wire 50 is cut by closing the wire clamp 73 and moving up the bonding head 70 in this state. Subsequently, a metal ball is formed at the tip of the wire 50 by arc discharge although not shown, and the process flow returns to the first process.
According to the aforementioned wire bonding method, there is no particular problem wherever the axis of the center of turn of the stem 200 exists since the bonding surface of the first bond and the bonding surface of the second bond make an angle of about 13xc2x0 between them. However, there is the problem that, if the angle of turn of the stem 200 is further increased, then the capillary 71 might be damaged by being brought in contact with the stem 200, the semiconductor laser element or the like, and as shown in FIG. 19, this leads to the problem that the metal wire 50 might be significantly bent at the tip of the capillary 71 or distorted and cut in the portions of the first bond and the tip of the capillary 71.
When die-bonding the Si sub-mount 260 to the heat radiation base 202 in the semiconductor laser device shown in FIG. 12, it is desirable to fix them with a conductive paste obtained by filling a resin with a conductive material (silver filler, for example) so as not to exert a thermal influence on the brazing material that fixes the semiconductor laser elements 231 and 232 to the Si sub-mount 260. However, there is the problem that the wire bonding cannot be performed when smoothness is lost due to the conductive paste adhering to the surface to which the metal wire is to be bonded, since the conductive paste has high liquidity and tends to spread over the die-bonding surface.
Accordingly, the object of the present invention is to provide a semiconductor laser device capable of simplifying the fabricating processes with a simple construction and easily mounting two semiconductor laser elements and a monitoring PD on a compact package and a wire bonding method for the above-mentioned semiconductor laser device capable of easily performing reliable wire bonding without damaging a stem, the semiconductor laser elements and so on.
In order to achieve the aforementioned object, the present invention provides a semiconductor laser device comprising:
a stem provided with a plurality of lead pins;
a sub-mount that is die-bonded onto the stem and has a surface formed integrally with a monitoring photodiode; and
two semiconductor laser elements that are die-bonded onto the sub-mount and have emission light monitored by the monitoring photodiode,
the semiconductor laser elements having electrodes electrically connected to the respective lead pins via metal wires and the monitoring photodiode having an electrode electrically connected to the corresponding lead pin via a metal wire, wherein
at least one first bonding surface of the two semiconductor laser elements and the monitoring photodiode is approximately perpendicular to a second bonding surface of the lead pin to be wire-bonded to the first bonding surface.
According to the semiconductor laser device having the above-mentioned construction, the electrodes of the two semiconductor laser elements and the electrode of the monitoring PD have mutually parallel electrode surfaces, and at least one of those three electrode surfaces is made to serve as a first bonding surface, which is wire-bonded to the second bonding surface of the lead pin approximately perpendicular to the first bonding surface. For example, in a small-size package having a diameter of 5.6 mm with a limited number of lead pins, the two semiconductor laser elements are arranged on the stem so that the optical axes of the emission light of the two semiconductor laser elements become parallel to each other and perpendicular to the stem surface (eyelet surface). If two lead pins exist on both sides of the direction of arrangement and another lead pin exists in a direction perpendicular to the direction of arrangement, then the electrodes of the semiconductor laser elements and the electrode of the monitoring PD are assigned to the three lead pins, and the electrodes and the lead pins are connected together by wire bonding (the other electrode of each element is connected to the stem that serves as a common electrode). In the above case, tangent planes on the peripheries of the lead pins on both sides of the direction of arrangement of the two semiconductor laser elements and two electrode surfaces out of the electrode of the semiconductor laser elements and the electrode of the monitoring PD become parallel to each other, allowing the wire bonding to be easily performed. However, the electrode surface (first bonding surface) of the remaining element, which is also parallel to a tangent plane on the periphery of the remaining lead pin, is wire-bonded to the end surface (second bonding surface) of the lead pin that is approximately perpendicular to the electrode surface (first bonding surface) of the remaining element. By thus enabling the wire bonding of the first and second bonding surfaces that are approximately perpendicular to each other, the fabricating processes can be simplified with a simple construction, and a semiconductor laser element capable of easily mounting the stem of a small-size package with two semiconductor laser elements and a monitoring PD can be provided. It is to be noted that a sub-mount to which the two semiconductor laser elements are to be die-bonded is provided by a sub-mount made of a semiconductor such as silicon so that a stress due to thermal expansion will not be applied to the semiconductor laser element.
In the semiconductor laser device of one embodiment, a bonding position of the first bonding surface and a bonding position of the second bonding surface are located in an identical plane approximately perpendicular to the first and second bonding surfaces.
According to the semiconductor laser device of the above embodiment, the bonding position of the first bonding surface and the bonding position of the second bonding surface are located in the identical plane approximately perpendicular to the first and second bonding surfaces. With this arrangement, the stem is turned along the identical plane in the wire bonding stage. Therefore, the metal wire is not twisted and no stress is applied to the semiconductor laser elements and the monitoring PD to which the metal wires are connected. Therefore, the reliability can be improved.
The semiconductor laser device of one embodiment further comprises metal lines, which are formed on the sub-mount and to which the two semiconductor laser elements are respectively die-bonded, wherein
the metal lines corresponding to the semiconductor laser elements are electrically insulated from each other.
According to the semiconductor laser device of the above embodiment, the metal lines, which are located on the sub-mount and to which the semiconductor laser elements are die-bonded, are independent metal lines provided for the respective semiconductor laser elements and electrically insulated from each other. This arrangement allows the two semiconductor laser elements to have different electrical characteristics on the die-bonding side. For example, it is acceptable to die-bond the p-electrode side of one semiconductor laser element and die-bond the n-electrode side of the other semiconductor laser element. Therefore, the conditions of the semiconductor laser elements to be employed have greater tolerance.
The semiconductor laser device of one embodiment further comprises metal lines, which are formed on the sub-mount and to which the two semiconductor laser elements are die-bonded, wherein
no metal line is formed from a rear end surface of at least one of the two semiconductor laser elements toward the monitoring photodiode.
According to the semiconductor laser device of the above embodiment, at least one of the metal lines is prevented from protruding from the semiconductor laser element toward the monitoring PD in the vicinity of the emission end surfaces of the semiconductor laser elements in order to make the largest amount of emission light from the semiconductor laser incident on the monitoring PD formed integrally with the sub-mount. This arrangement is effective particularly for the semiconductor laser element whose light emitting point is located several micrometers higher than the surface of the sub-mount.
In the semiconductor laser device of one embodiment, an end surface of the lead pin is the second bonding surface, and
the end surface of the lead pin is located at a height equal to a height of the surface of the stem or lower than the height of the surface of the stem.
According to the semiconductor laser device of the above embodiment, the capillary of the wire bonding apparatus can be prevented from striking against the lead pin that has the second bonding surface when performing wire bonding to the first bonding surface since the end surface of the lead pin, or the second bonding surface is located at the same height as that of the surface of the stem or lower than the surface of the stem.
In the semiconductor laser device of one embodiment, the stem is provided with stepped portions having bonding surfaces that are parallel to and different in height from a surface to which the sub-mount is bonded.
According to the semiconductor laser device of the above embodiment, the stem is provided with the stepped portions having the bonding surfaces that are parallel to and different in height from the surface to which the sub-mount is bonded. This arrangement eliminates the possibility of the occurrence of the problem that the wire bonding cannot be performed since the conductive paste on the stem surface to which the sub-mount is bonded does not adhere to the wire bonding surface.
The present invention also provides a semiconductor laser device comprising:
a stem provided with a plurality of lead pins;
a sub-mount die-bonded onto the stem; and
a semiconductor laser element die-bonded onto the sub-mount, the semiconductor laser element having an electrode electrically connected to the lead pin via a metal wire, wherein
the stem is provided with stepped portions having bonding surfaces that are parallel to and different in height from a surface to which the sub-mount is bonded.
According to the semiconductor laser device of the above embodiment, the stem is provided with the stepped portions having the bonding surfaces that are parallel to and different in height from the surface to which the sub-mount is bonded. This arrangement eliminates the possibility of the occurrence of the problem that the wire bonding cannot be performed since the conductive paste on the stem surface on which the sub-mount is bonded does not adhere to the wire bonding surface.
The present invention also provides a wire bonding method for a semiconductor laser device comprising a stem provided with a plurality of lead pins; a sub-mount that is mounted on the stem and has a surface formed integrally with a monitoring photodiode; and two semiconductor laser elements that are die-bonded onto the sub-mount and have emission light monitored by the monitoring photodiode, the method comprising:
a first step for retaining the stem so that an axis of a capillary for guiding a metal wire becomes perpendicular to at least one first bonding surface of the two semiconductor laser elements and the monitoring photodiode and bonding one end of the metal wire to the first bonding surface; and
a second step for turning the stem so that the axis of the capillary becomes perpendicular to a second bonding surface of the lead pin approximately perpendicular to the first bonding surface around an axis perpendicular to the metal wire after performing bonding of one end of the metal wire to the first bonding surface and bonding the other end of the metal wire to the second bonding surface.
According to the above semiconductor laser device wire bonding-method, the stem is retained so that the axis of the capillary of the wire bonding apparatus becomes perpendicular to at least one first bonding surface of the two semiconductor laser elements and the monitoring photodiode, and one end of the metal wire is bonded to the first bonding surface. Thereafter, the stem is turned so that the axis of the capillary becomes perpendicular to the second bonding surface of the lead pin approximately perpendicular to the first bonding surface around the axis perpendicular to the metal wire, and the other end of the metal wire is bonded to the second bonding surface. Through these processes, the metal wire can be connected to the first and second bonding surfaces that are about perpendicular to each other without twisting the metal wire to be bonded to the first and second bonding surfaces. Therefore, a semiconductor laser device capable of housing the two semiconductor laser elements and the monitoring PD in a small-size package can easily be subjected to wire bonding without damaging the stem, the semiconductor laser elements and so on.
According to the semiconductor laser device wire bonding method of one embodiment, the axis of turn of the stem in the second step is parallel to a line of intersection of the first and second bonding surfaces that are approximately perpendicular to each other.
According to the semiconductor laser device wire bonding method of the above embodiment, the axis of turn of the stem in the second step is made parallel to the line of intersection of the first and second bonding surfaces that are about perpendicular to each other, and the wire bonding is observed in the direction of the axis of turn of the stem. By this operation, the wire bonding can be performed while observing how the metal wire is twisted. Therefore, the bonding is not failed, and the wire bonding can be reliably performed.
According to the semiconductor laser device wire bonding method of one embodiment, a bonding position of the first bonding surface and a bonding position of the second bonding surface are located in an identical plane approximately perpendicular to the first and second bonding surfaces.
According to the semiconductor laser device wire bonding method of the above embodiment, the bonding position of the first bonding surface and the bonding position of the second bonding surface are located in the identical plane about perpendicular to the first and second bonding surfaces, and the stem is turned along the identical surface in the wire bonding stage. Therefore, the metal wire is not twisted, and no stress is applied to the semiconductor laser element and the monitoring PD to which the metal-wires are connected. Therefore, the reliability can be improved.
According to the semiconductor laser device wire bonding method of one embodiment, a distance from the axis around which the stem is turned in the second step to the first bonding surface is set equal to a distance from the axis around which the stem is turned to the second bonding surface.
According to the semiconductor laser device wire bonding method of the above embodiment, the distance from the axis around which the stem is turned to the first bonding surface is set equal to the distance from the axis to the second bonding surface. With this arrangement, the distances from the tip of the capillary to the first bonding surface and the second bonding surface becomes equal before and after the turn. Therefore, the wire bonding can easily be performed, and the bonded metal wire is hard to come off.
According to the semiconductor laser device wire bonding method of one embodiment, a length of the metal wire that is drawn out of a tip of the capillary by pulling up the capillary in a direction perpendicular to the first bonding surface after the first step is made longer than a length from a front end surface of the semiconductor laser element to a bonding position of the first bonding surface.
According to the semiconductor laser device wire bonding method of the above embodiment, the length of the metal wire to be drawn out of the tip of the capillary when the capillary is pulled up in the direction perpendicular to the first bonding surface after performing the bonding to the first bonding surface is made longer than the distance from the front end surface of the semiconductor laser element to the bonding position of the first bonding surface. With this arrangement, the capillary can be prevented from striking against the semiconductor laser element when the stem is turned.